1. Field of the Invention
The present invention relates to a thick film circuit board and a method of forming a wire bonding electrode thereon, particularly to a preferable substrate structure of a thick film circuit board where a gold (Au) wire are used as a bonding wire and a method of forming a wire bonding electrode for the gold wire.
2. Description of Related Art
According to a thick film circuit board used, for example, in an IC igniter, an IC regulator or the like of an engine control device, conductive wiring layers are normally printed and sintered on an insulating substrate such as a ceramic substrate or the like. Thick-film resistors each having a desired sheet resistance value are printed and sintered among the wiring layers thereby forming a thick film circuit. Thereafter, glass is printed and sintered as a protective film on the thick film circuit thus formed, as necessary. Finally, a processing of adjusting the resistances of the thick-film resistors to desired resistance values is performed by laser trimming.
In such a thick film circuit board, wiring layers comprising silver (Ag) have mainly been used as the conductive wiring layers. The reason is that silver (Ag) per se is excellent in workability and electric performance. Above all, silver was used significantly owing to the fact that the material of the thick film resistor was only a material, such as ruthenium oxide (RuO.sub.2), which could not be sintered under a nitrogen atmosphere. Further, ruthenium oxide (RuO.sub.2) per se is provided with an extremely excellent performance as the thick film resistor material.
However, in recent years, a thick film resistor material of tin oxide (SnO.sub.2) family or lanthanum boride (LaB.sub.6) family which can be sintered under a nitrogen atmosphere has newly been developed and wiring layers comprising further inexpensive copper (Cu) have mainly been used as the conductive wiring layers. The thick film resistor made of tin oxide (SnO.sub.2) family or lanthanum boride (LaB.sub.6) family has an extremely excellent performance as a resistor. Furthermore, in forming thick films, the thick films as well as copper (Cu) wiring layers can be sintered under a nitrogen atmosphere and therefore, the workability of a thick film circuit board as a whole is extremely excellent.
As mentioned above, in a thick film circuit board in recent years, a material of tin oxide (SnO.sub.2) family or lanthanum boride (LaB.sub.6) family is mainly used for a thick film resistor and a wiring layer comprising a copper (Cu) thick film is mainly used as a conductive wiring layer.
In mounting a semiconductor part on the insulating substrate of such a thick film circuit board, the mounted semiconductor part needs to be electrically connected to the conductive wiring layer through some means. As one of methods for electrically connecting the semiconductor parts with the conductive wiring layers, there is connection by wire bonding. In connection by wire bonding, it is regarded as the most preferable in view of reliability to use a gold (Au) wire as a bonding wire. Incidentally, it is known that when the gold (Au) wire is used as a bonding wire, even if diameter of the wire is reduced as fine as substantially 30 through 50 .mu.m, the wire is difficult to be cut up and further, even if the wire is brought under the severe environment such as high temperature or the like, its bonding performance is not deteriorated but rather improved.
However, the gold (Au) wire cannot directly be bonded to the copper (Cu) wiring layer which has been used in recent years as the conductive wiring layer. Accordingly, when the gold (Au) wire is used as a bonding wire, it is necessary to provide a land comprising a gold (Au) thick film also on the side of the insulating substrate in order to obtain a high reliable bonding. However, if all the conductive wiring layers on the board are formed by the gold (Au) thick film, there arises a problem that cost of the thick film circuit board increases, which is not preferable.
For that reason, as exemplified in FIG. 9A, it is conceivable that a gold (Au) thick film land 9 partially overlaps with a copper (Cu) wiring layer 2 that is formed as a thick film on an insulating substrate 1 of a ceramic substrate or the like film and a gold (Au) wire 8 is bonded to the gold (Au) thick film land 9.
However, in this case, it has been confirmed by the inventors that in the process of forming the gold (Au) thick film land 9, specifically in the process of printing gold (Au) thick film paste by a mode shown by FIG. 9A, drying it and thereafter, sintering it at temperatures of 600.degree. C. or higher, the gold (Au) thick film is cut up and disconnected from the copper (Cu) wiring layer 2 at an end of the copper (Cu) wiring layer 2 in a mode shown by FIG. 9B. This is presumed to be caused by the Kirkendoll phenomenon which occurs when gold (Au) in the gold (Au) thick film paste rapidly diffuses to the copper (Cu) thick film portion forming the copper (Cu) wiring layer 2. The Kirkendoll phenomenon is a phenomenon in which when two different metals are bonded by mutual diffusion, if there is a difference in diffusion coefficients inherent to the respective metal materials, cavities are caused on the side of a metal material having a larger value of the diffusion coefficient.
Conventionally, as a counter measure therefor, as exemplified in FIGS. 10A, 10B and 10C, the following structure has been proposed. After forming the copper (Cu) wiring layer 2 as a thick film on the insulating substrate 1 such as a ceramic substrate or the like (FIG. 10A), the gold (Au) thick film land 9 is formed at a position separated from the copper (Cu) wiring layer 2 by a pertinent gap G (FIG. 10B). Thereafter, a connecting portion 10 comprising nickel conductive paste is printed and sintered to partially overlap with each of the copper (Cu) wiring layer 2 and the gold (Au) thick film land 9 thereby electrically connecting the copper (Cu) wiring layer 2 and the gold (Au) thick film land 9 (FIG. 10C). The gold (Au) wire 8 is bonded to the gold (Au) thick film land 9 that is electrically connected to the copper (Cu) wiring layer 2 (FIG. 10C).
In this case, although the above-described disconnection is difficult to occur, the following problems arise. Because misregistration of the printed connecting portion 10, overflow of the nickel conductive paste and the like can happen, as shown by FIG. 10C, the connecting potion 10 needs to be formed larger than the copper (Cu) wiring layer 2 and the gold (Au) thick film land 9 by which flexibility of layout on the circuit board is significantly restricted. Further, the necessity of such a connecting portion 10 creates a hazard in downsizing of and high density arrangement on the circuit board. Moreover, the fabrication steps become complicated and an increase in cost is unavoidable.
Accordingly, the above-described structure involves many problems in practical application.